1. Field of the Invention
The present invention relates to a polishing apparatus and method for polishing a substrate in a process of planarizing the surface of a semiconductor wafer where a semiconductor device pattern is formed. Such a polishing apparatus is called a chemical mechanical polishing (CMP) apparatus.
2. Description of the Related Art
In a first prior art CMP apparatus (see JP-A-63-256356), a polishing platen associated with a polishing cloth (pad) thereon is rotated in one direction, and a polishing head is rotated in the same direction as that of the polishing platen.
Also, the back face of a semiconductor wafer is chucked to the bottom face of the polishing head. Therefore, the rotating polishing head with the semiconductor wafer is pushed onto the rotating polishing cloth while the rotating polishing head is rocking moving forward and backward in the horizontal direction. Thus, the front face of the semiconductor wafer can be flattened (planarized). This will be explained later in detail.
In the above-described first prior art CMP apparatus, however, since the polishing face of the semiconductor wafer is pushed onto the polishing cloth, it is impossible to observe the polishing face of the semiconductor wafer, so that an accurate control of thickness of the surface layer of the semiconductor wafer cannot be expected. Also, since the diameter of the polishing cloth is twice or more than that of the semiconductor wafer, most of the polishing liquid (abrasive) is dispersed by the centrifugal force due to the rotation of the polishing platen without contributing to the polishing of the semiconductor wafer, the utilization efficiency of the polishing liquid is low.
In a second prior art CMP apparatus (see JP-A-5-160088), a polishing platen for mounting a semiconductor wafer is rotated in one direction, and a polishing head associated with a polishing cloth thereon is rotated in the same direction as that of the polishing platen. In this case, the back face of the semiconductor wafer is checked to the face of the polishing platen. Also, the diameter of the polishing cloth is much smaller than that of the semiconductor wafer. Further, the polishing platen and the polishing cloth are rotated in the same direction. This also will be explained later in detail.
In the above-described second prior art CMP apparatus, however, since the diameter of the polishing cloth is much smaller that of the semiconductor wafer, the contact area of the polishing cloth to the semiconductor wafer W is very small, so that the polishing efficiency is very small.
Also, when the polishing cloth deviates from the edge of the semiconductor wafer, the contact area of the polishing cloth to the semiconductor wafer becomes small. As a result, the polishing speed in the edge of the semiconductor wafer increases.
Further, since the rotational direction of the polishing platen, i.e., the semiconductor wafer is the same as that of the polishing head, most of the polishing liquid is dispersed by the centrifugal force due to the polishing platen in addition to the centrifugal force due to the polishing head without contributing to the polishing of the semiconductor wafer, so that the utilization efficiency of the polishing liquid is low.
Additionally, since the polishing cloth is circular, the polishing power of the polishing cloth at its periphery is substantially increased.
Therefore, the polishing power is small at the center of the polishing cloth, while the polishing power is large at its periphery. Thus, it is difficult to homogenize the polishing power over the semiconductor wafer in spite of the rocking operation.
A third prior art CMP apparatus (see JP-A-7-88759), which also will be explained later in detail, also has the same problems as in the second prior art CMP apparatus.
It is an object of the present invention to provide a polishing apparatus and method having a large polishing efficiency, a suppressed polishing speed around the periphery of a semiconductor wafer (substrate), and high utilization of polishing liquid.
According to the present invention, in an apparatus for polishing a substrate, including a polishing platen for mounting the substrate thereon, a polishing head, a polishing pad adhered to a bottom face of the polishing head, and a rocking section, for rocking (moving) the polishing head in the horizontal direction with respect to the polishing platen, a control circuit controls a load of the polishing pad applied to the substrate in accordance with a contact area of the polishing pad to the substrate. Thus, the polishing pressure can be constant over the substrate.
Also, in a polishing method, a contact area of the polishing pad to the substrate is calculated. Then, a load of the polishing pad is calculated by multiplying the contact area of the polishing pad to the substrate by a contact polishing pressure. Finally, a load of the polishing pad is controlled in accordance with the calculated load of the polishing pad.